Evidence of Successions of Different Microbial Biota From MesoArchean South Africa

Sedimentology and facies analysis of Mesoarchaean stromatolitic carbonate rocks of the Pongola Supergroup, South Africa

Authors:

Shishi et al

Abstract:

The c. 3.0 Ga old Chobeni Formation (Nsuze Group, Pongola Supergroup) contains several carbonate successions interpreted to have been deposited in a tide-dominated, shallow-marine environment. Facies consist of mixed siliciclastic and carbonate rocks and include subtidal cross-bedded sandstone and oncolitic dolostone, intertidal wave-rippled oodolarenite, and supratidal wrinkly-laminated dolarenite–dololutite. Sedimentary facies are partially arranged in a shallowing-upward pattern reflecting cyclic variations of Mesoarchaean sea level. Microbialites occur in a variety of sedimentary sub-environments and include laminated as well as structureless, lenticular-domal types. Differences in microbialite morphology, macrofabric, and size suggest potentially distinct assemblages of microorganisms responding to different physicochemical and environmental conditions. Microbialite textures are consistent with in situ carbonate precipitation and to a lesser extent by trapping and binding of sediment. Despite their great age, the Pongola carbonates are compositionally and texturally very similar to much younger carbonate successions preserved in the geological record.

A Link Between the Cooler, Drier Aptian Cretaceous Climate and Carbonate Production Ending?

Cool episode and platform demise in the Early Aptian: New insights on the links between climate and carbonate production

Authors:

Bonin et al

Abstract:

The Early Aptian encountered several crises in neritic and pelagic carbonate production, major perturbations in the carbon cycle, and an Oceanic Anoxic Event (OAE1a). Yet the causal links between these perturbations and climate changes remain poorly understood, partly because temperature records spanning the Early Aptian interval are still scant. We present new δ18O data from well-preserved bivalves from a carbonate platform of the Galve sub-Basin (Spain) that document a major cooling event postdating most of OAE1a. Our data show that cooling postdates the global platform demise and cannot have triggered this event that occurred during the warmest interval. The warmest temperatures coincide with the time-equivalent of OAE1a and with platform biotic assemblages dominated by microbialites at Aliaga as well as on other tethyan platforms. Coral-dominated assemblages then replace microbialites during the subsequent cooling. Nannoconids are absent during most of the time-equivalent of the OAE1a, probably related to the well-known crisis affecting this group. Yet they present a transient recovery in the upper part of this interval with an increase in both size and abundance during the cool interval portion that postdates OAE1a. An evolution toward cooler and drier climatic conditions may have induced the regional change from microbial to coral assemblages as well as Nannoconids size and abundance increase by limiting continent derived input of nutrients.